Electric heating unit



Jan. 3, 1950 M. MCGRAW 2,493,542

ELECTRIC HEATING UNIT Filed July 29, 1947 49 it) INVENTOR.

/?74 Z; MA'X McGRAW ATT'Y with a range of from .03"

Patented-Jan. 3, 1950 UNITED s'rArEs PATENT OFFICE Max MeGnw, Chicago. Ill, mignor to McGraw Eleotrioh Company, Elgin, 11]., a corporation of ware.

' Application July 20, 1941, Serial No. 104,481 3 Claims. (Cl. 201-43) My invention relates to electric heating units and particularly to means for supporting the resistor in such units. r 1

An object of my'invention is toprovide an electric heating unit comprisingvery thin sheets of electric insulating material backed by metal sheets, the metal sheets having an electricinsulating coating thereon.

Another object of my invention is to provide an electric heater comprising a very thin sheet or sheets of mica unable to support the resistor by themselves, backed up by a metal sheet or sheets having on their entire outer surfaces an electricinsulating coating, the area of said metal sheets being substantiallythe same as that of the mica sheets.

Other objects of my invention will either be apparent from a description of several forms of electric heaters embodying my invention or will be pointed out in the course of such description and be set forth in the appended claims.

In the drawings, 7

Figure 1 is a view, in front elevation, of an electric heater embodying my invention,

Fig. 2 is a view in section therethrough, taken on the line 22 of Fig. 1,

Fig. 3 is a sectional view of a modification embodying my invention,

Fig. 4 is a sectional view flcation of electric heater embodying my invention, and,

Fig. 5 is a fragmentary view taken on the line 55 of Fig. 1.

Referring first of all to Figs. 1 and 2, I have there shown an electric heating unit, designated in its entirety by the numeral I I, which heating unit comprises two halves or parts [3 and I5. Each of the parts l3 and I5 comprises a relatively very thin sheet I! of mica or other electricinsulating material, which sheet may be of any desired contour and dimensions and is here shown as substantially rectangular in outline. The thickness of the mica sheet I! is on the order of .003" and while I have mentioned a particular thickness I do not desire to be limited thereto but may make the thickness of the mica sheets I! of any desired value ranging from .0025" to .0035".

I further provide a relatively thin metal sheet l9, which metal sheet may be of substantially the same contour and have substantially the same overall dimensions as those of the mica sheet. Preferably I use a thin aluminum sheet, the thickness of which is on the order of .035"

of a still further modi- I provide at each side of the units l3 and a relatively thick strip 2| of mica or or other electric-insulating material, having a thickness of about .035", which strip is provided with recesses 23 in its outer edge, which outer edge overhangs the outer edge of strips l1 and I9. The strips 2| are held in the desired and proper operative position by small rivets 25 extending through not only the strip 2| but also through the sheets I! and I3.

I provide a resistor strip 21' which is wound through the recesses 23 and across each of the sheets I! and is substantially as shown in Fig. 1 of the drawings. The two heating unit portions l3 and I5 may be held in spaced adjacent positions by relatively thick short strips 29 of electricinsulating material, such as mica, having a thickness of about .035", being positioned against the outer surface of the sheets iii of aluminum and at each side edge thereof.

I provide a hollow metal rivet 3| in the lower portion of part is, as well as a similar rivet 3| in the lower portion of part l3, to which the ends of the resistor strip may be connected.

While I may make the resistor conductor 21 of any of the usual resistance materials, I prefer to use an aluminum resistor and provide the entire outer surface of resistance conductor 21 with an integral, inorganic, heat-conducting, high-temperature-resisting and electric-insulating coating, the thickness of this coating being on the order of .001". While I have mentioned a. specific thickness of coating I do not desire to be limited thereto since the thickness of the coating may vary from .0005 to .0015"; U. S. Patent No. 1,526,127 discloses one method of making such an electric-insulating coating.

The entire outer surface of the sheets ill of aluminum is also provided with a coating of substantially the same kind as was hereinbefore set forth in connection with resistor conductor 21.

Referring now to Fig. 3 of the drawings, I have there shown a modified form of electric heater 33 comprising a single sheet 35 of a suitable electric-insulating material, the thickness of which is substantially the same as was hereinbefore set forth for sheets "in Fig. 1, and I provide also a sheet 31 of aluminum. The thickness of the aluminum sheet may be of substantially the same degree as was hereinbefore set forth in connection with sheets IQ of Fig. 1.

I provide an insulator strip 39, preferably of mica or the like, which is relatively heavy so that a resistance conductor 4! may be wrapped in recesses 43 in the side strip ll in the same manner as was hereinabove set forth in connection with the recesses 23, the resistance conductor ll being adapted to extend across that side of the heating unit upon which the mica sheet 35 is located.

Rivets 45 are adapted to hold the parts 35, 31 and 39 in proper operative positions relative to each other. r

Referring now to Fig. 4 of the drawings, I have there illustrated a heating unit 4'! which is adapted to be provided with a double winding, namely-one in which the resistance conductor extends along both sides of the heating unit. I provide sheets 49 of electric-insulating material, preferably mica, spaced apart by a sheet of aluminum. The sheets 49 are of a thickness on the order of .003" with the same allowable variation as liereinbefore set forth in connection with sheets I! of Fig. 1, while the thickness of sheet 5| of aluminum is on the order of .035" with substantially the same allowable variation of thicknesses was hereinbefore set forth in connection with sheets l9 of Fig. 1.

I provide a pair of strips 53 of electric-insulation, preferably mica, which strip is thick enough (about .035") to receive a plurality of recesses 55 in its outer edge, within which recesses a resistor 51 may be wound to extend across the sheets 49 and 51 from one side to the other and, as has already been set forth, the resistance conductor 51 is adapted to be wound around the heating unit in a substantially helical form.

I have hereinbefore set forth that the thickness of the sheet of electric-insulation is about .003" and while the insulating value of .003" thickness of the electric-insulation is suflicient, its mechanical strength is insufficient to support a resistance conductor 21, 4! or 5'1, particularly when it is considered that the resistance conductor must be wound upon the very thin sheet of electric-insulation.

In order to provide the sheet of electric-insulation with sufficient mechanical strength to withstand both its handling during the manufacture of the heating unit as well as during the life thereof, I provide a sheet of metal, which sheet of metal is provided over its entire outer surface with an electric-insulating coating, the thickness of which is about .001".

To further ensure a heating unit construction in which the resistance of the sheet of electricinsulation is relied upon mainly to electrically insulate the resistance conductor, I mount the resistance conductor on that side of the assembled sheets of electric-insulation and of aluminum upon which is found the sheet of electricinsulation. The electric-insulating value of the coatin on the aluminum sheet is, however, sufficient to withstand breakdown of the electricinsulation sheet and short circuiting of parts of the resistor winding so that I have provided a doubled insulation for the resistor.

The structure shown in Figs. 1 and 2 may be used in the heating units of electric toasters, while the structures shown in Figs. 3 and 4 inelusive may be used in any form of electric heater with which the structure embodying my invention may be used.

Various modifications may be made in the device embodying my invention without departing from the spirit and scope thereof and I desire that all such modifications coming clearly within the scope of the appended claims shall be covered thereby.

I claim as my invention:

1. An electric heating unit comprising a sheet of electric-insulating material too thin to constitute the sole support of an electric resistor, a sheet of metal of light weight and large heattransmitting characteristics, a relatively thick strip of electric-insulating material secured to said two sheets at each of two opposite edges thereof, recesses in the outer edges of said strips and a resistor member supported by said recesses in said strips extending across the outer surface of said sheet of electric-insulating material.

2. An electric heating unit comprising a sheet of electric insulating material too thin to constitute the sole support of an electric resistor, a thin metal backing sheet lying against one surface of said insulating sheet, a relatively thick strip of electric insulating material secured to said two sheets at each of two opposite edges thereof and extending along those edges, said thick strips extending forward of the surface of said insulating sheet, recesses in the outer edges of said strips, and an elongated resistor strung from strip to strip and lying in said recesses and supported thereby across the outer surface of said insulating sheet and spaced therefrom.

3. An electric heating unit comprising a sheet of electric insulating material too thin to constitute the sole support of an electric resistor, a thin metal backing sheet lying against one surface of said insulating sheet, a relatively thick spacing strip of electric insulating material lying against the face of said insulating sheet along each of two opposite edges thereof, means for securing said spacing strips to both of said sheets and an elongated resistor strung between said strips and supported thereby spaced from the exposed face of said insulating sheet.

MAX McGRAW.

REFERENCFlS orrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 494,291 Dewey Mar. 28, 1893 1,332,852 Levy Mar. 2, 1920 FOREIGN PATENTS Number Country Date 12,702 Great Britain June 1, 1916 434,635 Great Britain Sept. 5, 1935 471,579 Great Britain Sept. 7, 1937 

